magit-commit-assert: bugfixes
magit-read-char-case: use read-char-choice
magit-branch-delete: cosmetics
magit-format-log-margin: cosmetics
magit-diff-select-algorithm: cosmetics
magit-diff-popup: improve doc-string, cosmetics

Well "improve" is a bit of a strong word here; "make it like
the doc-strings of other popups" describes it more truthfully.
magit-diff-paths: bugfix

The fix is actually in `magit-diff-refresh-buffer';
do not append "--" if ARGS already contains that.